Small quakes at Parkfield provide clues to big ones

February 17, 1995

By Diane LaMacchia, DMLaMacchia@LBL.gov

Data from a network of seismic sensors along the Parkfield stretch of the San
Andreas fault is providing important insight into the nature of earthquake
cycles.

Earth scientists from LBL and UC Berkeley report that small earthquakes, which
have been steadily occurring along the central 25-km stretch of the Parkfield
fault zone (northeast of San Luis Obispo), are increasing in frequency and
depth and are moving outside of their characteristic clusters.

"For the first time," says Thomas McEvilly of LBL's Earth Sciences Division,
"we are seeing something systematic in the process that we presume is the
nucleation [initiation] of a magnitude six earthquake."

At the San Andreas fault, two great slabs of the Earth's crust--the Pacific
plate and the North American plate--generate seismic activity as they slip past
each other. The Parkfield fault zone is the subject of a number of scientific
studies because of the magnitude-six earthquakes that have occurred there
regularly for more than a hundred years. Based on historical records from 1857
to 1966, the quakes typically happen every 20 to 30 years, but can come as
early as 12 years or as late as 32 years after the previous one. Another
magnitude-six quake is expected at Parkfield within the next few years.

Since 1987, scientists from LBL and UCB have run a precision high-resolution
seismic network at Parkfield. It involves 10 boreholes, each 200 to 300 meters
deep, and containing a set of three sensors cemented at the bottom. With these
sensors, several thousand seismic events have been recorded over the years. The
results have been analyzed at LBL's Center for Computational Seismology.

In the January 27 issue of Science, Robert Nadeau, William Foxall, and
McEvilly reported their analysis of phenomena observed between 1987 and 1992.
The study is part of Nadeau's dissertation.

They observed that only a small fraction of the fault zone was responding to
the motion of the plates; almost two-thirds of the several thousand earthquakes
recorded occurred in less than one percent of the active fault zone. The
earthquakes were clustered in approximately 300 small cells, each about 20
meters across, unevenly distributed along the fault. When one cell was
activated, another would go off as far away as about 200 meters, implying some
sort of communication process between the two cells, such as fluid migration in
the rocks.

The small earthquakes measure less than magnitude one on the Richter scale and
recur with distinct regularity. "These micro-earthquakes are indistinguishable
from one to the next--they're identical," McEvilly says. "Most of them occur
with a very regular periodicity, of the order of one year. They're like seismic
pulsars."

The regularity of the small quakes at Parkfield supports the idea that seismic
activity is not chaotic.

"Here's a laboratory experiment that demonstrates that at this magnitude range
repeating characteristic earthquakes do occur and at this scale are fairly
predictable," McEvilly says.

In the course of the study, conducted during what is thought to be the final
portion of the interval between the last big earthquake at Parkfield and the
one due to come, the frequency of small earthquakes has increased steadily,
from fewer than 200 earthquakes per year to 500. At the same time, they are
happening deeper in the earth, and fewer of them reside in the cluster cells.
According to the scientists, these changes in seismic activity may be signals
of an impending magnitude-six quake.